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Calculation details
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Calculation report
Includes formulas, assumptions, limitations and identified sources.
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Quick answer#
The rebar calculator creates a straight-bar quantity for a rectangular grid or a set of parallel runs. It reports base length, order length, whole stock bars and estimated weight using the spacing, cover, layers, allowance and bar weight entered by the user.
Rebar grid formula#
Clear length equals the slab dimension minus twice the entered edge cover. Bars running along the slab length are spaced across the clear width; bars running across the width are spaced along the clear length.
For either direction, bar count = round clear perpendicular dimension ÷ maximum spacing upward + 1. Total grid length is both bar counts multiplied by their corresponding clear run lengths and by the number of layers.
Worked example#
For a 20 ft × 10 ft slab with 3 in edge cover and 12 in maximum spacing, one layer contains 11 bars running nearly 20 ft and 21 bars running nearly 10 ft. The exact base length is approximately 414 ft. A 10% allowance produces about 455.4 ft, which rounds to 23 stock bars at 20 ft each.
Rebar sizes and weight#
The size selector provides common U.S. bar designations with their conventional linear weights. ASTM A615/A615M covers deformed and plain carbon-steel bars for concrete reinforcement and includes standard size designations and dimensions.
The calculator keeps weight per length editable. Use the specification, mill or supplier value when the selected material, coating, standard or market differs.
Allowance, laps and stock bars#
The percentage input is not an automatic splice design. It is one transparent planning allowance for laps, cuts and ordering. Actual development and lap lengths depend on design conditions, bar properties, concrete, cover, spacing, location and applicable requirements.
Stock-bar count divides total order length by one entered stock length and rounds upward. It does not optimize a cutting list or account for mandatory splice positions.
Assumptions and limitations#
This tool does not design reinforcement, choose bar size or spacing, verify cover, calculate development length, produce bends or hooks, create a bar schedule, check constructability or determine code compliance. Use approved drawings and qualified structural review.
Sources and methodology#
- ASTM A615/A615M — specification scope and standard reinforcing-bar size designations.
- Concrete Reinforcing Steel Institute — reinforcing-steel practice and reference context.
- NIST SP 811 conversion factors — U.S. customary and SI conversions.
These sources identify measurement and product standards. They do not supply a project reinforcement design.